For measuring instruments having a rotatable measuring mechanism it is known to eliminate bearing friction by using taut strip suspended or tension band mounted measuring mechanisms. Previously such a taut strip suspension or tension band mounting was realized only in galvanometers and similar highly sensitive laboratory instruments due to their high mechanical sensitivity as regards shock and vibration and in view of their position dependency. With suitable structural measures for fastening the taut strips and for absorbing shocks and vibrations on the one hand and with improvements of the properties of the tape materials on the other hand, the mounting has now been perfected to such a degree that it can be used successfully today even in shock-resistant switchboards.
A taut strip or tension band must perform three functions:
1. IT FORMS THE BEARING FOR THE MEASURING MECHANISM;
2. IT PROVIDES CURRENT TO THE MEASURING MECHANISM; AND
3. IT IMPARTS THE REQUIRED RESETTING MOMENT TO THE MEASURING MECHANISM.
The requirements placed on the taut strip due to the simultaneous performance of these three functions are very high and often exclude one another due to their opposite nature. As a bearing, the strip must have the highest tensile strength without being brittle in order to be able to withstand shocks and vibrations. In the interest of high sensitivity, a small reset moment is required. This means that materials for taut strips must have as high as possible a breaking strength factor .sigma..sub.B on the one hand and as low as possible a modulus of torsion G. The suitability of a material for tensioning belts is thus characterized by a parameter or quality factor Z which is equal to the breaking strength divided by the square root of the modulus of torsion of the respective material.
In addition to the breaking strength and torsion modulus, the tension band material must be corrosion resistant, workable and easily solderable. For the production of high precision measuring instruments it is additionally important that the elastic after-effect and hysteresis be as low as possible. In certain cases in which low or normal demands are placed on the breaking strength of the taut strip suspension, it is desirable to reduce at least the torsion modulus as much as possible.
Heretofore, platinum/iridium alloys with an iridium content up to 30%, platinum/nickel alloys with a nickel content of 8 to 12.5% and gold/nickel alloys have been preferred for the taut strips. Alloys of the platinum metals (platinum, palladium, rhodium) with 5 to 40% iron, cobalt, nickel, tungsten, molybdenum, copper or silver and 1 to 30% iridium are suggested for fabrication of tension bands in German Pat. No. 1,152,826. Such alloys, according to this patent, exhibit torsion moduli in the order of 6300 with quality factors of about 2.50.